A radio (wireless) communication system may be comprised of an access network and a plurality of access terminals. The access network may include access points, such as Node Bs, base stations, or the like, that allow the access terminals to connect with the access network for uplink (UL: terminal-to-network) communications and downlink (DL: network-to-terminal) communications via various types of channels. The access terminals may be user equipment (UE), mobile stations, or the like.
Although the concepts described hereafter may be applicable to different types of communication systems, the Universal Mobile Telecommunications System (UMTS) will be described merely for exemplary purposes. A typical UMTS has at least one core network (CN) connected with at least one UTRAN (UMTS Terrestrial Radio Access Network) that has Node Bs acting as access points for multiple UEs.
FIG. 1 shows the radio interface protocol architecture according to the 3GPP radio access network standards. The radio interface protocol has horizontal layers comprising a physical layer, a data link layer, and a network layer, and has vertical planes comprising a user plane (U-plane) for transmitting user data and a control plane (C-plane) for transmitting control information. The user plane is a region that handles traffic information with the user, such as voice or Internet protocol (IP) packets. The control plane is a region that handles control information for an interface with a network, maintenance and management of a call, and the like.
The protocol layers in FIG. 1 can be divided into a first layer (L1), a second layer (L2), and a third layer (L3) based on the three lower layers of an open system inter-connection (OSI) standard model. The first layer (L1), namely, the physical layer (PHY), provides an information transfer service to an upper layer by using various radio transmission techniques. The physical layer is connected to an upper layer called a medium access control (MAC) layer, via a transport channel. The MAC layer and the physical layer exchange data via the transport channel. The second layer (L2) includes a MAC layer, a radio link control (RLC) layer, a broadcast/multicast control (BMC) layer, and a packet data convergence protocol (PDCP) layer. The MAC layer handles mapping between logical channels and transport channels and provides allocation of the MAC parameters for allocation and re-allocation of radio resources. The MAC layer is connected to an upper layer called the radio link control (RLC) layer, via a logical channel. Various logical channels are provided according to the type of information transmitted.
The MAC layer is connected to the physical layer by transport channels and can be divided into a MAC-b sub-layer, a MAC-d sub-layer, a MAC-c/sh sub-layer, a MAC-hs sub-layer and a MAC-m sub-layer according to the type of transport channel being managed. The MAC-b sub-layer manages a BCH (Broadcast Channel), which is a transport channel handling the broadcasting of system information. The MAC-c/sh sub-layer manages a common transport channel, such as a forward access channel (FACH) or a downlink shared channel (DSCH), which is shared by a plurality of terminals, or in the uplink, the Random Access Channel (RACH). The MAC-m sub-layer may handle the MBMS data. The MAC-d sub-layer manages a dedicated channel (DCH), which is a dedicated transport channel for a specific terminal. The MAC-d sub-layer is located in a serving RNC (SRNC) that manages a corresponding terminal and one MAC-d sub-layer also exists in each terminal.
The RLC layer, depending of the RLC mode of operation, supports reliable data transmissions and performs segmentation and concatenation on a plurality of RLC service data units (SDUs) delivered from an upper layer. When the RLC layer receives the RLC SDUs from the upper layer, the RLC layer adjusts the size of each RLC SDU in an appropriate manner based upon processing capacity, and then creates data units by adding header information thereto. These data units, called protocol data units (PDUs), are transferred to the MAC layer via a logical channel. The RLC layer includes a RLC buffer for storing the RLC SDUs and/or the RLC PDUs.
The BMC layer schedules a cell broadcast (CB) message transferred from the core network and broadcasts the CB message to terminals positioned in a specific cell or cells.
The PDCP layer is located above the RLC layer. The PDCP layer is used to transmit network protocol data, such as IPv4 or IPv6, efficiently on a radio interface with a relatively small bandwidth. For this purpose, the PDCP layer reduces un-necessary control information used in a wired network, namely, a function called header compression is performed.
The radio resource control (RRC) layer located at the lowest portion of the third layer (L3) is only defined in the control plane. The RRC layer controls the transport channels and the physical channels in relation to setup, reconfiguration, and the release or cancellation of the radio bearers (RBs). The RB signifies a service provided by the second layer (L2) for data transmission between the terminal and the UTRAN. In general, the set up of the RB refers to the process of stipulating the characteristics of a protocol layer and a channel required for providing a specific data service, and setting the respective detailed parameters and operation methods. Additionally, the RRC layer handles user mobility within the RAN, and additional services, e.g., location services.
The E-UTRA (Evolved UMTS Terrestrial Radio Access) system, also called a LTE (Long Term Evolution) system, is considered as involving the PS (Packet Switched) domain with only shared resources to be used. In this new context with faster delay and higher capacity requirements, the usage of LTE RACH (LTE Random Access Channel) should be somewhat different to the existing GSM and UMTS systems in order to meet access requirement specified for LTE. E-UTRA and LTE are related to the principles of Orthogonal Frequency Division Multiplexing (OFDM).
OFDM is based on the well-known technique of Frequency Division Multiplexing (FDM). In FDM, different streams of information are mapped onto separate parallel frequency channels. Each FDM channel is separated from the others by a frequency guard band to reduce interference between adjacent channels. The OFDM technique differs from traditional FDM in the ways that multiple carriers (called sub-carriers) carry the information stream, the sub-carriers are orthogonal to each other; (i.e. the bandwidths of the individual sub-carriers, are small and arranged so that the maximum of one carrier, corresponds with the first minimum of the adjacent carrier) and a guard time may be added to each symbol to combat the channel delay spread.
FIG. 2 shows an exemplary Frequency-Time representation of an OFDM signal. As can be seen, the signal may be comprised of multiple sub-carriers, each sub-carrier (having a particular bandwidth or frequency range) may carry data (or information) that are represented by symbols with guard intervals therebetween.
The multi-user system comprises both uplinks and downlinks. In the uplink, the network measures the attenuation at the different uplink sub-carriers. On the basis of the measurements made, the network distributes the sub-carriers which the different UEs have to use for uplink transmission. In the downlink, the UE measures the attenuation for each downlink sub-carrier. The result of the measurement is signaled to the network which distributes downlink sub-carriers for better UE reception. In a random access protocol, a UE transmits a known signal sequence (i.e. a specific coded signature) to a base station (Node B). For that, firstly, the UE listens for a pilot channel transmitted by the network, and after detection, the UE synchronizes to OFDM symbols transmitted by the network. Secondly, the UE listens to a broadcast system information channel for a random access sequence and a sub-carrier number assigned to a random access channel (RACH) and then transmits a random access sequence in the random access channel. After transmission for a number of cycles of the random access sequence, the UE checks whether or not, the network has granted the access.
A general overview of the W-CDMA random access procedure will now be considered.
The transport channel RACH and two physical channels PRACH and AICH, are involved in this procedure. The transport channels are the channels supplied from the physical layer to the protocol layer (MAC). There are several types of transport channels to transmit data with different properties and transmission formats over the physical layer. Physical channels are identified by code and frequency in FDD mode. They are normally based on a layer configuration of radio frames and timeslots. The form of radio frames and timeslots depends on the symbol rate of the physical channel. The radio frame is the minimum unit in the decoding process, consisting of 15 time slots. The time slot is the minimum unit in the Layer 1 bit sequence. Thus, the number of bits that can be accommodated in one time slot depends on the physical channel. The transport channel RACH (Random Access Channel) is an uplink common channel used for transmitting control information and user data. It is applied in random access, and used for low-rate data transmissions from the higher layer. The RACH is mapped to the uplink physical channel called the PRACH (Physical Random Access Channel). The AICH (Acquisition Indication Channel) is a downlink common channel, which exists as a pair with the PRACH used for random access control.
The transmission of PRACH is based on a slotted ALOHA approach with fast acquisition indication. The UE selects randomly an access resource and transmits a RACH preamble part of a random access procedure to the network. The preamble is a short signal that is sent before the transmission of the RACH connection request message. The UE repeatedly transmits the preamble by increasing the transmission power every time the preamble is sent, until the UE receives an AI (Acquisition Indicator) on the AICH (Acquisition Indicator Channel), which indicates the detection of the preamble by the network. The UE stops the transmission of the preamble once it receives the AI, and sends the message part at the level of power equal to the preamble transmission power at that point, plus an offset signalled by the network. This random access procedure avoids a power ramping procedure for the entire message. Such power ramping procedure would create more interference due to unsuccessfully sent messages and it would be less efficient due to the larger delay since it would take much more time to decode the message before an acknowledgement could be given that it was received successfully.
The main characteristics of the RACH is that it is a contention based channel, which means that due to simultaneous access of several users, collisions may occur such that the initial access message cannot be decoded by the network. The UE can start the random-access transmission (both preambles and message) at the beginning of an access slot only. This kind of access method is therefore a type of slotted ALOHA approach with fast acquisition indication.
FIG. 3 shows an example of access slots in relation to the transmission of a preamble, a message, and an acquisition indicator (AI).
FIG. 4 shows an example of the number of RACH access slots and their spacing.
Referring to FIGS. 3 and 4, the time axis of both the RACH and the AICH is divided into time intervals, called access slots. There are 15 access slots per two frames (one frame is 10 ms length or 38400 chips) and they are spaced 1.33 ms (5120 chips) apart.
FIG. 5 shows an example of reception of DL AICH and UL PRACH by a UE and the transmission of uplink PRACH access slot by the UL. Namely, FIG. 5 shows the transmission timing relationship between the PRACH and AICH.
FIG. 6 shows a table with the available uplink access slots for different RACH sub-channels.
Referring to FIGS. 5 and 6, the information on what access slots are available for random-access transmission and what timing offsets to use between RACH and AICH, between two successive preambles and between the last preamble and the message is signalled by the network. For example, if the AICH transmission timing is 0 or 1, it will be sent 3 or 4 access slots after the last preamble access slot transmitted, respectively.
Referring to FIGS. 5 and 6, the timing at which the UE can send the preamble is divided by random access sub-channels. A random access sub-channel is a subset comprising the combination of all uplink access slots. There are 12 random access sub channels in total. Random access sub-channel consists of the access slots.
FIG. 7 shows an exemplary format of preamble signatures. The preamble is a short signal that is sent before the transmission of the RACH message. A preamble consists of 4096 chips, which is a sequence of 256 repetitions of Hadamard codes of length 16 and scrambling codes assigned from the upper layer. The Hadamard codes are referred to as signature of the preamble. There are 16 different signatures and a signature is randomly selected (from available signatures sets on the basis of ASC) and repeated 256 time for each transmission of preamble part.
FIG. 8 shows an exemplary structure of a random access message part. The message part is spread by short codes of OVSF codes that are uniquely defined by the preamble signature and the spreading codes as the ones used for the preamble signature. The message part radio frame of length 10 ms is divided into 15 slots, each consisting of 2560 chips. Each slot consists of a data part and a control part that transmits control information (pilot bits and TFCI). The data part and the control part are transmitted in parallel. The 20-ms-long message part consists of two consecutive message part radio frames. The data part consists of 10*2 k bits (k=0, 1, 2, 3), which corresponds to the Spreading Factor (SF=256, 128, 64, 32).
FIG. 9 shows an exemplary format (structure) of the AICH. The AICH consists of a repeated sequence of 15 consecutive access slots, each of length 40 bit intervals (5120 chips). Each access slot consists of two parts, an Acquisition Indicator (AI) part consisting of 32 real-valued signals a0, . . . , a31 and a part of duration 1024 chips where transmission is switched off.
When the network detects transmission of an RACH preamble in an RACH access slot with a certain signature, it repeats this signature in the associated AICH access slot. This means that the Hadamard code used as signature on RACH preamble is modulated onto the AI part of the AICH. The acquisition indicator corresponding to signature can take the values +1, −1, and 0, depending on whether a positive acknowledgement a negative acknowledgement or no acknowledgement is given to a specific signature. The positive polarity of signature indicates that the preamble has been acquired and the message can be sent. The negative polarity indicates that the preamble has been acquired and the power ramping procedure shall be stopped, but the message shall not be sent. This negative acknowledgement is used when, due to a congestion situation in the network, a transmitted message cannot not be processed at the present time. In this case, the access attempt needs to be repeated some time later by the UE.
Regarding the random access procedure on protocol layer (L2), the network decides whether the mobile station is to be permitted use of a radio access resource based primarily upon the access class to which the UE belongs. A specified priority level is implied by the Access Class (AC) which is stored on the UE SIM card.
Hereafter, certain aspect of access control will be described. It should be noted that the relevant standard related to this matter is 3GPP TS 22.011.
Regarding the purpose of access control, under certain circumstances, it will be desirable to prevent UE users from making access attempts (including emergency call attempts) or responding to pages in specified areas of a PLMN (Public Land Mobile Network). Such situations may arise during states of emergency, or where 1 of 2 or more co-located PLMNs has failed. Broadcast messages should be available on a cell-by-cell basis indicating the class(es) of subscribers barred from network access. The use of this facility allows the network operator to prevent overload of the access channel under critical conditions. It is not intended that access control be used under normal operating conditions.
For allocation, all UEs are members of one out of ten randomly allocated mobile populations, defined as Access Classes 0 to 9. The population number can be stored in a SIM/USIM for the UE. In addition, mobiles may be members of one or more out of 5 special categories (Access Classes 11 to 15), which also may be stored in the SIM/USIM. These may be allocated to specific high priority users as follows. (This enumeration is not meant as a priority sequence):
Class 15—PLMN Staff;
Class 14—Emergency Services;
Class 13—Public Utilities (e.g. water/gas suppliers);
Class 12—Security Services;
Class 11—For PLMN Use.
For operation, if the UE is a member of at least one Access Class which corresponds to the permitted classes as signalled over the air interface, and the Access Class is applicable in the serving network, access attempts are allowed. Otherwise access attempts are not allowed.
Access Classes are applicable as follows:
Classes 0˜9—Home and Visited PLMNs;
Classes 11 and 15—Home PLMN only;
Classes 12, 13, 14—Home PLMN and visited PLMNs of home country only.
Any number of these classes may be barred at any one time.
For emergency calls, an additional control bit known as Access Class 10 is also signalled over the air interface to the UE. This indicates whether or not network access for Emergency Calls is allowed for UEs with access classes 0 to 9 or without an IMSI. For UEs with access classes 11 to 15, Emergency Calls are not allowed if both Access Class 10 and the relevant Access Class (11 to 15) are barred. Otherwise, Emergency Calls may be allowed.
Hereafter, the mapping of Access Classes (AC) will be described. It should be noted that the relevant standard related to this matter is 3GPP TS 25.331.
In UMTS, the Access Classes are mapped to Access Service Classes (ASC). There are eight different priority levels defined (ASC 0 to ASC 7), with level 0 being the highest priority.
For mapping of Access Classes to Access Service Classes, the Access Classes shall only be applied at initial access, i.e. when sending an RRC CONNECTION REQUEST message. A mapping between Access Class (AC) and Access Service Class (ASC) shall be indicated by the information element AC-to-ASC mapping in System Information Block type 5. The correspondence between AC and ASC is indicated in FIG. 10.
FIG. 10 shows a table showing the correspondence between AC and ASC. The no IE designates an ASC number i in the range 0-7 to AC. If the ASC indicated by the no IE is undefined, the UE behaviour is unspecified.
For random access, the parameters implied by the respective ASC shall be employed. In case the UE is a member of several ACs, it shall select the ASC for the highest AC number. In connected mode, AC shall not be applied.
An ASC consists of a subset of RACH preamble signatures and access slots which are allowed to be used for this access attempt and a persistence value corresponding to a probability Pv≦1 to attempt a transmission. Another important mechanism to control random access transmission is load control mechanism which allows reducing the load of incoming traffic when the collision probability is high or when the radio resources are low.